2.1. Introduction: Apelin and the Apelin Receptor (APJ)
The apelin receptor (APJ) was cloned in 1993 as an orphan G-protein coupled receptor (GPCR). The human APJ gene is located on the long arm of chromosome 11 and encodes a 377 amino acid G protein-coupled receptor. The gene for APJ was designated angiotensin-receptor like 1 (AGTRL1) due to sequence similarities between the two receptors. Carpene et al., J Physiol Biochem. 2007; 63(4):359-373. However, none of the known peptidergic ligands for the angiotensin receptors, including angiotensin, activate APJ. APJ remained an orphan GPCR until 1998 when the peptide apelin was identified as its endogenous ligand. Lee et al., J Neurochem. 2000; 74(1):34-41; Habata et al., Biochim Biophys Acta. 1999; 1452(1):25-35.
Over the years, apelin and APJ have emerged as an important regulator of various physiological processes. Both apelin and APJ are expressed in the central nervous system (CNS) and peripherally in a number of tissues. Expression of APJ has been noted within the vasculature of some organs and is a potent regulator of related processes including angiogenesis and vasoconstriction. Cobellis et al. report increased of expression levels of both apelin and APJ receptor in preeclampsia-complicated pregnancies. Cobellis et al., Histol Histopathol. 2007; 22(1):1-8. APJ is also expressed in nonvascular cell types in heart, liver, and CNS where its primary role is currently under investigation. Medhurst et al., J Neurochem. 2003; 84(5):1162-1172. Apelin and APJ are often co-localized within the same organ suggesting an autocrine regulation of the receptor by its ligand. However, apelin has since been detected in blood suggesting that concomitant paracrine regulation of the receptor is also possible. The apelin-APJ system has been implicated as a regulator of various physiological functions and is believed to play an important role in thermoregulation, immunity, glucose metabolism, angiogenesis, fluid homeostasis, cardiac function, hepatic function and renal function. Ladeiras-Lopes et al., Arq Bras Cardiol. 2008; 90(5):343-349. APJ also acts as a co-receptor during HIV infection. O'Donnell et al., J Neurochem. 2007; 102(6):1905-1917; Zou et al., FEBS Lett. 2000; 473(1):15-18.
Expression of apelin and APJ are either up- or down-regulated in various pathophysiological conditions. In particular, the APJ appears to be an emerging target for the treatment of cardiovascular failure, liver fibrosis, cancer, angiopathies, pancreatitis, and as a prophylactic against HIV infection. In 2011 Andersen et al. reviewed apelin and APJ as an opportunity for therapeutic uses for pulmonary hypertension and pulmonary arterial hypertension (PAH). Andersen et al. Pulm. Circ. 2011; 1(3) 334-346.
Unfortunately, small molecule ligands of the APJ having suitable pharmacological properties are lacking. Few nonpeptide ligand systems has been reported to date. Iturrioz et al. report compounds that contain polycyclic fluorophores, such as lissamine, which make them ill-suited for pharmaceutical uses. Iturrioz et al., FASEB J. 2010; 24:1506-1517; EP 1903052 (Llorens-Cortes et al.). US Publ. Pat. Appn. 2014/0094450 (Hachtel et al.) discloses benzoimidazole-carboxylic acid amide derivatives as APJ receptor modulators.
Accordingly, there is a need for small molecule agonists of APJ.